1. Field of the Invention
The present invention relates to a facsimile apparatus, and in particular to a facsimile apparatus that includes a modulating/demodulating device that can be driven at a low voltage.
2. Related Background Art
There are some conventional modulating/demodulating devices that are driven at high voltages, such as +12 V to -12 V, or +5 V to -12 V, for example. In a facsimile apparatus that employs such a modulating/demodulating device, a peripheral or external circuit (hereafter referred to as a "peripheral circuit") that transmits a signal to, or receives a signal from, an external device is provided on the outside of the modulating/demodulating device. Since the drive voltage for the modulating/demodulating circuit is high, for the transmission of 0 dB a signal from the modulating/demodulating circuit is transmitted externally without being amplified by the peripheral circuit. For reception of 0 dB, a signal from the external device is received by the modulating/demodulating circuit without being amplified by the peripheral circuit.
Recently, in consonance with the high scale circuit integration, such as LSI, defects occur due to the heat generated in a circuit. To eliminate the defect, various highly integrated circuits have been produced, including a modulating/demodulating circuit that is driven at a low voltage, 0 to +5 V, for example. A modulating/demodulating device that is driven by a single power source of +5 V mainly tends to be employed.
Some facsimile apparatuses employ such a modulating/demodulating device that can be driven by a single power source that has a low voltage output.
Since a signal received from, or transmitted to, a modulating/demodulating device that can be driven by a low voltage falls within a voltage range having a maximum limit of 3.5 Vp-p, amplification and attenuation processing by the peripheral circuit is required. When the transmission of 0 dB is to be performed where the prevailing conditions involve a transfer mode of 9600 b/s, the performance of QAM modulation, the provision of an internal 7.2 km cable equalizer in the modulating/demodulating device, and a constant amplification rate for the peripheral circuit, a signal must be sent to a circuit at a level of about 8 Vp-p. Thus, the peripheral circuit has to increase the level by about 7.2 dB. This increased level value is acquired by the following expression: EQU 20 log(8/3.5)=7.2(dB).
Since the amplification rate of the peripheral circuit is a constant value, to transmit a signal of -8.0 dB, for example, the modulating/demodulating device outputs an attenuated signal of 1.39 Vp-p (1.39 Vp-p is acquired as x in 20 log(x/8.0)=-15.2). Accordingly, the modulating/demodulating device performs attenuation and the peripheral circuit thereafter performs amplification, which is a wasteful process.
To perform the setting for the receipt of a signal of 0 dB, attenuation of approximately 7.2 dB (=20 log(8.0/3.5)) must be performed by the peripheral circuit. As the attenuation rate of the peripheral circuit is constant, however, when a signal of -40 dB is received, the input level to the modulating/demodulating device is reduced to -47.2 (=-40-7.2) dB and the S/N ratio is accordingly decreased.
In a facsimile apparatus that employs a modulating/demodulating device that can be driven at a low voltage, the S/N ratio will be reduced by a process whereby for transmission a modulating/demodulating device attenuates a signal and thereafter a peripheral circuit amplifies the resultant signal, or a process whereby for reception the peripheral circuit attenuates a received signal and thereafter transmits the resultant signal to the modulating/demodulating circuit, even though the modulating/demodulating circuit can receive a signal directly.